Aortic stenosis is the obstruction of outflow from the left ventricular chamber into the aorta caused by a restricted opening of the aortic valve during left ventricular contraction and ejection. The diminished aortic valve opening area (from normally more than 3 cm2 in an averaged sized adult to less than 0.5 cm2 in severe cases) results in a significant pressure drop across the valve, and normal cardiac output and aortic pressure can only be maintained at the expense of an increased left ventricular systolic pressure. The high intracavitary pressure which has to be generated by the left ventricular chamber results in increased wall tension and myocardial oxygen demand. Adaptive processes such as hypertrophy (compensatory increase in muscle mass) allow the heart to withstand this increased pressure load for some time, but ultimately, pump failure is inevitable.
In the majority of cases (and in more than 90% of all patients older than 65 years) aortic stenosis is caused by progressive fibrous and calcified degeneration of an originally normal valve, a process which is favored by hyperlipoproteinemia, arterial hypertension, and aging (acquired calcified aortic stenosis). The average survival of a patient with severe aortic stenosis and shortness of breath is less than two years. Since death may occur suddenly in a substantial portion of cases, some investigators recommend preventive surgery even in asymptomatic patients, provided they are good surgical candidates.
Surgical results in the selected group of patients with isolated aortic stenosis are reasonable. Operative mortality in such patients is about 5%. However, most individuals with significant aortic stenosis are in their seventies and eighties. These patients have usually multiple comorbid risk factors, such as coronary artery disease, cerebrovascular disease, generalized atherosclerosis, renal failure, or diabetes. Consequently, surgical mortality and morbidity are substantial. Moreover, if the calcified aortic valve is replaced by a mechanical prosthesis, anticoagulation is mandatory to reduce thromboembolic complications, which exposes the patient to an increased risk of serious bleeding, particularly with increasing age. Implantation of biological prostheses is therefore usually preferred in the elderly, but surgically implanted biological valves may have a suboptimal hemodynamic profile, because the suture ring on which the valve needs to be mounted reduces the space available for the valve itself. This poses a particular problem in women, where bioprostheses of a smaller size (which have to be used because of the smaller cardiac dimensions) may result in significant residual outflow obstruction.
Because of the significant risk of elderly patients undergoing open-heart surgery on cardiopulmonary bypass, which includes death, disabling stroke, respiratory and renal complications, dilatation of the narrowed valve using balloon-catheters was hoped to provide an alternative to surgery. Unfortunately, because immediate results of the balloon dilatation are suboptimal, and recoil of the stenosis reoccurs within weeks and months in virtually all patients, outcome is as poor as in patients who do not undergo surgery. Balloon-dilatation is therefore considered only justified in patients with a clear contraindication to surgery or—in rare cases—as a “bridging procedure”.
Recently, in analogy to the use of stents in coronary arteries, it has been proposed to use valved stents in order to achieve a sufficiently large valve area and avoid elastic recoil and restenosis. Spencer et al (U.S. Pat. No. 6,730,118), Andersen et al (U.S. Pat. No. 5,840,081) and Gabbay (U.S. Pat. No. 4,759,758) all describe a valved stent of certain designs that are intended for transarterial deployment. Cribier et al describes, in WO 98/29057, a collapsible stent which has a valve attached to it by circumferential suturing. The mesh/valve system is deployed via an inflatable balloon. In 1992, Andersen et al. reported their experience with a foldable porcine aortic valve sutured in an expandable stainless-steel stent. The valved stent was mounted on an 18–22 mm balloon-catheter front-loaded in a 16F Mullins long sheath and implanted in the pulmonary position, completely displacing the pulmonary cusps (or leaflets), which were pressed between stent and pulmonary artery wall with full deployment of the stent. However, this approach could result in coronary artery occlusion when undertaken in the aortic position, which would be fatal to the patient.
Even when the stent is not deployed across the full area of the aortic annulus, atheromatous deposits on the ventricular side of the aortic cusps (or leaflets) may be pushed against the ostia of the coronary arteries causing severe coronary obstruction or embolization. Severe distention of a heavily calcified aortic valve to allow deployment of a sizeable stent may also cause embolization of calcium deposits from the valve or a tear in the valve resulting in significant aortic regurgitation. Furthermore, a large stent-valve may also interfere with surrounding structures such as the anterior mitral leaflet (causing damage to it or impairing its function), and if protruding into the left ventricular outflow tract, the basal ventricular septum, which is usually hypertrophied in significant aortic stenosis.